(1) Field of the Invention
This invention is directed to the field of acoustic velocimetry, and more particularly, to a non-invasive acoustic velocimeter and a non-invasive method of acoustic velocimetry.
(2) Description of the Prior Art
In submarine sonar applications, vessels are presently detected using active sonar where an acoustic pulse of a given frequency or frequencies is reflected off the target vessel and returns to the sonar system at essentially the same frequency or frequencies. Accurate knowledge of the speed of sound in the marine environment is necessary to obtain accurate positioning from active sonar. The speed of sound in a fluid is dependent on the density of the fluid which is related to its temperature, salinity, and pressure. Various inhomogeneities and interfering reflections can also limit the performance of active sonar systems. Consequently, it is desirable to determine a profile of the velocity of sound at various points within the marine environment to obtain accurate sonar readings.
Conventional acoustic or sonar remote sensing velocimeters typically use an invasive technique wherein a velocimeter having an acoustic source and a receiving unit in close proximity to each other is lowered on a cable to the water depth where the sound velocity is to be measured. Sound velocity is determined by measuring the travel time between the source and receiver while the velocimeter is being lowered or raised. The velocimeter known in the prior art requires physical lowering of the measuring unit to the desired depth. One of the problems created by physical lowering of the measuring unit is the time required to deploy the device, lower it to the required depth, raise the device back to the location of the vessel, and stow the device aboard the vessel, These tasks become even more difficult if the deploying vessel is a submarine. Furthermore, the deploying vessel must be moved to obtain velocimetric readings along a horizontal profile.